Constant-intensity-light sources are ubiquitous in optical sensing systems used in medical, environmental, and industrial applications. Temperature stability of these optical sources is essential as it is the foundation of the instrument accuracy, be it through calibration at manufacturing time or in real time correction. Optical and electrical properties of all light emitting components and optical materials vary with temperature. For this reason an absolutely stable source does not exist. Rather, sources of varying stability are developed for specific applications. In many applications today, where lasers are needed, the control of laser output intensity is realized by stabilizing the laser's temperature which in turns stabilizes its wavelength and output. Using thermo-electric-coolers (TEC) raises the cost of such laser sources and lowers their wall-plug efficiency. In many applications, for example, actuators for sensors or machine vision, stable intensity and low power consumption are important, while stable emission wavelength is not critical. In this case, uncooled source may be used in combination with sophisticated intensity compensation that makes the source immune to temperature changes over some temperature range and source degradation. Uncooled sources are preferred in systems where size and cost are important.
The stabilization of the output beam intensity from lasers and light-emitting diodes is conventionally performed using a closed-loop control system illustrated in FIG. 1 (PRIOR ART). A light source 101, which may be an incandescent lamp, a light-emitting diode, or a laser, is powered by control electronic 106 and emits a collimated light beam 105 onto a beam splitter 102. The transmitted portion of the beam 103 is useful light that may be used for illumination or measurement/sensing. The reflected portion 104 of the incident light 105 is captured by the photodetector 107. The intensity of the reflected beam 104 is converted to electric current 108 in the photodetector 107. The control electronics 106 compares the reflected beam 104 intensity in form of current 108 against a reference 110.
The primary factors producing intensity drift in the output beam intensity versus temperature are (a) temperature drift in the reference 110, (b) the change in the emission wavelength or emission spectra of the light source with temperature, which in turn can change the transmittance to reflectivity ratio of the beam-splitter, and (c) scattered light reaching the detector and offsetting the measured power is also temperature dependent. If the optical source 101 degrades with time, the control electronics 106 will make necessary correction in optical source 101 output intensity 103.
Constant-intensity optical sources based on lasers are known in the field. For example United States patent application 20120025714 by Downing and Babic discloses uses a vertical-cavity surface emitting laser and a weakly polarizing optical interference coating to achieve high stability over temperature and time. The operation of the invention disclosed relies on the temperature drift in the emission wavelength of single-mode linear-polarization-locked lasers. The fine adjustment of the output power temperature drift coefficient is adjusted by rotating the laser around the optical axis in order to adjust the beam's polarization. An important factor coming to play with a coherent optical source such as a single-mode laser is the appearance of interference fringes appearing on transparent objects external to the source. This is particularly problematic if the beam is used to measure the properties of liquids stored in containers with transparent walls. If interference is a problem in the measurement setup, there is an advantage in using incoherent optical sources. Light-emitting diodes have coherence length which is significantly smaller than the thickness of the walls on most glass pipes and test vials. For this reason, interference fringes rarely occur in the measurement.
Light-emitting diodes are used to provide constant light output with reliability, accuracy, and power efficiency that surpass incandescent lamps used in similar instruments historically. However, due to wavelength drift with temperature, high degree of intensity stability (better than 100 ppm/° C.) is not achievable due to the temperature variation in the systems components with wavelength.
An additional problem in realizing ultra-stable optical sources is the manufacturing tolerance of components that directly influence the stability response, such as, variation in detector responsivity, manufacturing variation in the interference coating characteristics (if used), or temperature dependence of the comparator at input to the control circuit.
This application discloses several embodiments of an optical source based on light-emitting diodes or lasers that feature high temperature stability of the output-beam intensity.
One of the objectives of this application is to disclose a highly temperature-stable uncooled optical source of incoherent light. Another objective is to disclose a highly temperature-stable uncooled optical source of coherent polarized light.